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Effect of ultrasonic agitation on NiCo and NiFe deposition
Surface Technology, 22 (1984) 219 - 239
219
E F F E C T OF ULTRASONIC AGITATION ON N i - C o AND N i - F e DEPOSITION T. R. MAHMOOD
State Organization for Technical Industries, Baghdad (Iraq) J. K. DENNIS and P. L. B A R R E T T
Department of Metallurgy and Materials Engineering, University of Aston in Birmingham, Birmingham B4 7ET (Gt. Britain) (Received January 16, 1984)
Summary The effects of ultrasonic agitation on deposition from two iron group alloy plating solutions, Ni-Co and bright Ni-Fe, have been studied. Comparison has been made with deposits plated from the same solutions using controlled air agitation. The ultrasonic equipment employed had a fixed frequency of 13 kHz b u t the power o u t p u t from each transducer was variable up to a maximum of 350 W. The effects of air and ultrasonic agitation on hardness, ductility, tensile strength and composition have been studied. The results obtained show that by increasing the ultrasonic power the hardness of both alloys was increased significantly and their compositions changed. The cobalt content of Ni-Co was decreased and the iron content of N i - F e increased. The ductility of coatings was increased by use of ultrasonic agitation b u t the tensile strength did not change very much. Ultrasonic agitation is an expensive means of agitating plating solutions and is worthwhile only if significant improvements in properties can be achieved. The novel feature of simultaneous improvement in hardness and ductility could have useful engineering applications.
1. Introduction The application of ultrasonic agitation in electroplating has received attention in recent years because of its effects on grain size, hardness, internal stress, brightness, porosity and reduction in plating time [1]. Relevant literature in this field provides conflicting results, probably due to the methods b y which ultrasonics are applied and because different frequencies and intensities have been used which makes comparison of results difficult. Most attention has been paid to single-metal deposits and only limited information has been published on the behaviour of alloy plating solutions. 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
220 Some of the early claims for the effects of ultrasonic agitation were rather exaggerated but it has been demonstrated that some definite advantages may be gained by using high frequency sound during the electrodeposition of alloys. These include deposition at higher than normal current densities, resulting in decreased plating times, improvement in physical properties [2], increased brightness, higher adhesion to the substrate, finer grain size, reduced internal stress and lower porosity. It has been found that ultrasonic fields can increase the rate of metal ion transfer within the electrolyte solution and so modify the properties of the deposit. The intensity and distribution of the sound field within a plating cell play an important role in determining the effects observed. It is the aim of the present investigation to study the effects of ultrasonic agitation during electrodeposition of two nickel alloys and to determine whether worthwhile improvements can be achieved.
2. Experimental procedure
2.1. Equipment The plating cell consisted of a plastic tank of volume 10 1. This was a suitable size to enable it to fit into the ultrasonic tank unit. The cathode panel 10 cm × 5 cm was clamped midway between the two anodes placed at either end of the bath. A three-phase rectified d.c. supply was used. Two series of experiments were carried out, one using ultrasonic agitation and the other using air agitation. The ultrasonic equipment consisted of two main parts: (i) a frequency generator; (ii) a tank unit. The frequency generator gave a m a x i m u m o u t p u t of 350 W per transducer at a frequency of 13 kHz. The transducers could be tuned and the output power varied. The tank unit consisted of a free-standing container housing two magnetostrictive transducers fixed in the b o t t o m of a stainless steel tank. Ledges were provided inside the tank to enable the use of a sheet of stainless steel as a platform on which to stand the plating cell. It was assumed after discussions with the manufacturers that a 15% power loss occurred. The plating cell was immersed in water to approximately three-quarters of its height in the ultrasonic tank.
2.2. Electroplating procedure Flat mild steel panels 10 cm × 5 cm were plated in nickel, cobalt, NiCo and Ni-Fe solutions in the plastic cell using ultrasonic agitation. The airagitated nickel, cobalt and Ni-Co panels were also plated in the same cell but the air-agitated N i - F e samples were plated in a 55 1 tank as described below. Most panels were plated at 4 A dm -z for 40 min and the rest at 8 A dm -2 for 20 min. These panels were used for both hardness determination and analysis.
221 Flat brass strip-type Hounsfield test pieces were plated in both alloy plating solutions in the 10 1 tank. Both modes of agitation were used and all samples were plated for 40 min at 4 A dm -2.
2.3. Nickel plating solution The solution used for the electrodeposition of nickel was based on the Watts bath. The solution was purified before use by treating with 20 g of activated carbon per litre to remove organic impurities and by plating at low current density (0.5 - 1.0 A dm -2) for several hours to remove metallic contamination. All other baths were treated in a similar manner. 2.4. Cobalt plating solution The bath used for the electrodeposition of cobalt was based on the Watts nickel-type bath, i.e. the nickel c o n t e n t was replaced by cobalt salts. The solution composition was as follows: cobaltous sulphate (COSO4- 7HEO), 150 g 1-1; cobaltous chloride (COC12"6H20), 22.5 g l-i; boric acid (H3BO3) , 1 5 g l 1. 2.5. Ni-Co sulphate-chloride plating solution The mixed sulphate-chloride bath for the deposition of Ni-Co is well known, being based on the Watts nickel bath. The composition of the solution chosen for the initial work was as follows: nickel sulphate (NiSO4" 7H:O), 290 g 1 1; nickel chloride (NiCl:'6H20), 40 g l-l; COSO4"7H20, 10 g 1-1, H3BO3, 40 g 1-1. Rolled nickel anodes were used and the concentration of cobalt in the solution was maintained by addition of COSO4" 7H20. In order to vary the cobalt c o n t e n t of deposits, further plating solutions were prepared with 35, 60, 85 and 110 g COSO4" 7H20 1-1. 2.6. Ni-Fe plating solution A proprietary bright-levelling Ni-Fe solution was prepared as recommended by the manufacturers. The solution chosen for the work was an N i - F e alloy plating solution consisting of the following: NiSO4" 7H20, 130 g 1-1; NiC12.6H20 , 100 g 1-1; H3BO3, 40 g 1-1; FeSOa. 7H20 , 12.5 g 1-1; complexing agent, 20 g l-l; saccharin, 2 g 1-1; initial brightener (proprietary solution of wetting agent, saccharin leveller and brightener), 10 ml 1-1. In addition to the standard steel panels plated in the 10 1 cell, others were plated in a 55 1 tank of N i - F e solution using vigorous air agitation. The reason for plating in this tank rather than the 10 1 tank was to keep the solution in o p t i m u m working condition. Not only was it possible to keep the metal ion concentration constant but by regular plating it was also possible to prevent excessive formation of ferric iron. The pH value of this solution was maintained within the range 3.8 - 4.2; this range was dictated by the need to keep the trivalent iron at a low level, since a high rate of formation of iron(III) was favoured by a high pH. If the
222 pH fell below 3.8 levelling was reduced. Nickel carbonate was used to raise the pH to its r e c o m m e n d e d value. If the. pH rose above 4.2 there was a loss of brightness in the low current density areas and the ductility was reduced. The pH was lowered to t he r e c o m m e n d e d value when necessary using dilute 20 vol.% sulphuric acid. The solution t e m p e r a t u r e was maintained at 68 °C because at temperatures below 63 °C there was a reduction in brightness, levelling and cat hode current efficiency. Temperatures above 70 °C caused harmful degradation p r o d u c t s to form as a result of stabilizer breakdown.
2. 7. Maintenance o f the bath The iron was present in the bath as ferrous (iron(II)) and ferric (iron (III)) ions. It was i m p o r t a n t that the ferric iron c o n c e n t r a t i o n did not exceed 40% o f the total iron c o n c e n t r a t i o n in the bath, otherwise loss of ductility and cath o d e current efficiency resulted. If the bath was operated at pH values higher than r e c o m m e n d e d , the ferric iron c o n c e n t r a t i o n had a tend e n c y to rise. Under normal operating conditions, the ferric iron concentration did n o t exceed 20% of the total iron in solution because ferric iron was constantly being reduced to ferrous at t he cathode. The brightening and levelling properties of the solution were maintained by regular additions of a single maintenance brightener at the recomm e n d e d rate. 2.8. Anodes 7 5 w t . % N i - 2 5 w t . % F e alloy in granular form was contained in a titanium anode basket which was enclosed in a well-washed c o t t o n anode bag. 2.9. Hardness Microhardness tests were carried out on m o u n t e d cross sections of the deposits cut from the centre of the mild steel panels. Ten readings were taken from each sample and the average was calculated. 2.10. Tensile tests The widths and thicknesses of specimens were measured before and after plating. A nominal 5 cm gauge length was scribed on t h e m and the edges c h a m f e r e d to remove the thick coating f r o m these high current density regions so that p r e m a t u r e edge cracking did n o t occur. The exact distance between the scribed crosses was measured accurately by a Vernier travelling microscope. The specimens were fixed into the grips of a Hounsfield t e n s o m e t e r and th e strain was applied manually. The elongation and load were recorded. An initial m e a s u r e m e n t was taken shortly after the appearance o f small cracks or o t h e r tiny defects in the deposit. The test piece was unloaded and the p e r m a n e n t elongation of t he gauge length measured together with the applied load. The specimen was reloaded and e x t e n d e d until final failure occurred. The final elongation was calculated and the m a x i m u m load recorded. Triplicate results were obtained in all cases.
223
2.11. Analysis The compositions of the deposits produced by the two agitation methods were determined by electron probe microanalysis, atomic absorption spectrophotometry or wet chemical analysis. 3. Results
3. I. Composition and hardness 3.1.1. Ni-Co deposits The effects of ultrasonic agitation on the average hardness and the cobalt c o n t e n t are shown in Figs. 1 and 2. After every 3 h of plating the Ni-Co solution was analysed by atomic absorption spectrophotometry and then replenished by adding COSO4- 7H20 to bring it back to its original concentration. Detailed examination of the results [3] showed that the hardness and cobalt c o n t e n t of the air-agitated deposits were approximately the same at 4 and 8 A dm -2. The hardness at 4 A dm -2 varied between 254 and 280 HV, and at 8 A dm -2 between 258 and 282 HV. The cobalt c o n t e n t at 4 A dm -2 was between 19.6% and 26.5% while at 8 A dm -2 it was between 19.4% and 24.8%. The deposits obtained from the Ni-Co solution using ultrasonic agitation showed that the hardnesses and cobalt contents of the deposits depend on the degree of ultrasonic agitation power and current density. At 4 A dm 2 the cobalt content in the deposit varied from 22.6% to 18.4% whilst the hardness varied from 265 to 345 HV because different power intensities (from 20 to 350 W) were transmitted to the 10 1 electroplating bath, but at
cg I /
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ULTRASONIC
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AGITATION
. . . .
3~0
'
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POWER , Watts
Fig. 1. Variation in hardness o f N i - C o alloys with degree o f ultrasonic agitation (the plating solution contained 10 g COSO4"7H20 1-1): --e--, current density of 4 A dm-2; -- © --, current density of 8 A dm -2.
224 23-
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2b 4b 160 ULTRASONIC
260 AGITATION P O W E R ,
3b0 Watts
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Fig. 2. Variation in Ni-Co alloy composition with degree of ultrasonic agitation (the plating solution contained 10 g CoSO4"7H20 l-i): ~ - - , current density o f 8 A din-2; -- o --, current density of 4 A dm -~.
SO
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Fig. 3. Relation between the COSO4"7H20 concentration in solution and the cobalt content in the deposit: - - o - - , air agitation;--$--, ultrasonic agitation (350 W).
8 A d m -2 the c o b a l t c o n t e n t in t h e d e p o s i t varied f r o m 22.00% to 18.00% w h i l e the hardness varied f r o m 2 6 9 t o 3 5 8 H V . F r o m these results it is e v i d e n t that ultrasonic agitation had a far greater e f f e c t o n hardness than o n c o b a l t c o n t e n t in t h e d e p o s i t , a l t h o u g h there w a s greater scatter in t h e hardness results using ultrasonic agitation. T h e average hardness for air-agitated s a m p l e s at 4 A d m 2 was 2 6 5 . 6 H V and for t h e highest u l t r a s o n i c p o w e r o f 3 5 0 W at 4 A d m -2 was 3 4 5 HV.
225
Since air agitation gave an average cobalt c o n t e n t in the deposit of 23.3% and an average hardness of 265.6 HV whilst ultrasonic agitation power of 350 W gave an average cobalt content of 18.4% and an average hardness of 345.3 HV, it seems reasonable to assume that the increase in hardness is due to the ultrasonic agitation effect.
3.1.2. Effect o f change in solution composition Further work was carried out using plating solutions with different cobalt concentrations. It was apparent t h a t the small percentages of cobalt in the bath resulted in high percentages of cobalt in the electrodeposit. The relation between the cobalt contents in the deposits and the concentration of COSO4- 7H20 in solution is shown in Fig. 3.
3.1.3. Ni-Co solution containing 110 g COS04" 7H20 1-1 The cobalt c o n t e n t of the deposit using air agitation was initially 81.8% and dropped to 68.3% at 350 W ultrasonic agitation. The hardnesses obtained using different ultrasonic agitation intensities ranged from 296 to 383 HV. Deposits with hardnesses of 308 HV and 312 HV were obtained by applying vigorous air agitation at 4 A dm -2 and 8 A dm -2 respectively. The more intense was the ultrasonic agitation the harder were the electrodeposits, as shown in Fig. 4, and the greater the intensity of the ultrasonic agitation the lower was the cobalt content, after an initial rise at 40 W, as shown in Table 1. A comparison between the hardness results obtained from this solution and those from that containing 10 g COSO4" 7H20 1-1 using ultrasonic agitation is also illustrated in Fig. 4. 380-
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270• 260250 0 20 4b
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Fig. 4. V a r i a t i o n in h a r d n e s s o f nickel, c o b a l t a n d N i - C o alloys w i t h degree o f u l t r a s o n i c a g i t a t i o n : - - © --, N i - C o (110 g COSO4"7H20 l-S); - - ~3 - - N i - C o (10 g COSO4"7H20 1-1); ---o--, c o b a l t s o l u t i o n ; - - " - - , dull nickel s o l u t i o n .
226 TABLE 1 Hardness and composition of Ni-Co alloy deposits obtained using air or ultrasonic agitation (pH 4 ; temperature, 55 °C) Agitation
Current density (A dm 2)
Co content (%)
Hardness (HV)
Air Air Ultrasonic Ultrasonic Ultrasonic Ultrasonic Ultrasonic
4 8 4 4 4 4 4
81.8 81.2 64.8 72.4 71.5 69.5 68.3
308 312 297 296 308 328 383
20 W 40 W 100 W 200 W 350 W
The plating solution contained 110 g
C o S O 4 ° 7H20
1-1.
3.1.4. N i c k e l d e p o s i t s F r o m t h e r e s u l t s s h o w n in T a b l e 2 it is a p p a r e n t t h a t s o m e c o b a l t w a s p r e s e n t as i m p u r i t y . E l e c t r o d e p o s i t e d n i c k e l r a n g e d in h a r d n e s s f r o m 2 4 7 t o 253 HV when deposited using vigorous air agitation and current densities of 4 a n d 8 A d m -2. H a r d e r d e p o s i t s , a b o u t 2 5 9 - 3 1 2 H V , w e r e o b t a i n e d u s i n g u l t r a s o n i c a g i t a t i o n o f d i f f e r e n t p o w e r i n t e n s i t i e s . A v e r a g e h a r d n e s s versus u l t r a s o n i c a g i t a t i o n p o w e r is i l l u s t r a t e d in F i g . 4.
TABLE 2 Hardness and composition of nickel electrodeposits obtained using air or ultrasonic agitation (pH 4; temperature, 55 °C; current density, 4 A dm -2 unless otherwise stated) Agitation
Ni content (%)
Co content (%)
Hardness (HV)
Air Air 8 A dm -2 Ultrasonic 20 W Ultrasonic 40 W Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 350 W
94.47 92.16 94.61 94.02 94.09 94.19 94.31
5.88 8.32 5.62 6.10 6.30 5.86 5.70
247 253 258.6 264.8 265.4 275 312
3.1.5. Cobalt d e p o s i t s The hardnesses of the electrodeposits obtained by using air agitation w e r e 2 8 4 H V a n d 2 8 7 H V u s i n g c u r r e n t d e n s i t i e s o f 4 A d m - 2 a n d 8 A d m -2 r e s p e c t i v e l y . A t 4 A d m -2 t h e h a r d n e s s r a n g e d f r o m 2 9 2 . 6 t o 3 6 1 . 9 H V u s i n g u l t r a s o n i c a g i t a t i o n o f d i f f e r e n t p o w e r i n t e n s i t i e s , as s h o w n in T a b l e 3 a n d F i g . 4.
227 TABLE 3 Hardness of cobalt electrodeposits obtained using air or ultrasonic agitation (pH 4;temperature, 55 °C) A g i t a tio n
Air Air Ultrasonic Ultrasonic Ultrasonic Ultrasonic Ultrasonic
20 W 40 W 100 W 200 W 350 W
3.1.6. Ni-Fe
Cu rre n t density
Co content
Ni content
(A dm -2)
(%)
(%)
4 8 4 4 4 4 4
99.87 99.70 99.58 99.68 99.70 99.68 99.65
0.21 0.33 0.44 0.43 0.35 0.41 0.45
Hardness
(HV) 284 287 294.6 292.6 304 322 361.9
deposits
T h e h a r d n e s s e s o f t h e N i - F e alloy e l e c t r o d e p o s i t s r a n g e d f r o m 523 t o 5 3 8 . 8 H V using v i g o r o u s air a g i t a t i o n a n d c u r r e n t densities o f 4 a n d 8 A d m -2. H a r d e r d e p o s i t s w e r e o b t a i n e d ( 5 2 7 . 3 - 6 2 3 . 4 H V ) using u l t r a s o n i c a g i t a t i o n o f increasing p o w e r i n t e n s i t y , as s h o w n in Fig. 5. T h e iron c o n t e n t o f t h e d e p o s i t using air a g i t a t i o n was 24.8%. W h e n u l t r a s o n i c a g i t a t i o n was a p p l i e d t h e iron c o n t e n t varied b e t w e e n 1 9 . 4 2 % at 20 W a n d 26% a t 3 5 0 W. T h e s a m p l e s w e r e a n a l y s e d b y a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y a n d e l e c t r o n p r o b e m i c r o a n a l y s i s . T h e results illustrated in Fig. 6 s h o w g o o d a g r e e m e n t .
600
5 0 0 - 1
0 26 4b
.
.
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ULTRASONIC
.
.
.
2bo
AGITATION
.
.
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-
.
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POWER , Watts
Fig. 5. Variation in hardness of Ni-Fe alloys with degree of ultrasonie agitation.
228
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Fig, 6. Variation in N i - F e alloy composition with degree of ultrasonic a g i t a t i o n : - ©--, results determined by electron probe microanalysis; ---$--, results determined by atomic absorption spectrophotometry.
3.2. Tensile strength and ductility 3.2.1. Ni-Co The average results are given in Tables 4 and 5. Figure 7 shows a typical load-extension curve for a brass specimen, 0AB, and for an electroplated brass specimen, 0C. During the testing of the Ni-Co electrodeposited specimens, fracture was always preceded by cracking of the deposit and fracture usually started from one of the edge defects. Thus it appears probable that fracture initiates in the Ni-Co electrodeposit and if this is completely ruptured before failure starts in the brass the load carried by the brass will be no longer equivalent to AD but to CD, which is greater than the ultimate tensile strength (UTS) of the brass. The brass will then fail without further TABLE 4 Average tensile strength and ductility of test pieces plated with an Ni-Co coating obtained from a solution containing 10 g COSO4"7H20 1-1 and uncoated brass
Method of agitation
Average load at failure of plated test piece
Average UTS of Average true UTS plated test of plated test piece piece
(kN)
(kN mm -2)
(kN mm -2)
0.320 0.328 0.328 0.330 0.317
0.464 0.474 0.483 0.490 0.463
Air 7.10 Ultrasonic 100 W 7.32 Ultrasonic 200 W 7.31 Ultrasonic 350 W 7.33 Unplated brass 6.80 UTS, ultimate tensile strength.
Average elongation (%) 37.1 41.2 42.5 45.5 69.9
229 TABLE 5 Average tensile s t r e n g t h a n d ductility o f p l a t e d N i - C o coatings o b t a i n e d f r o m a s o l u t i o n c o n t a i n i n g 10 g COSO4"7H20 1-1 d e t e r m i n e d in situ o n brass s u b s t r a t e s
Method o f agitation
First cracks Average load (kN)
Air 5.43 Ultrasonic 100 W 5.76 Ultrasonic 200 W 5.88 Ultrasonic 350 W 5.94
Average stress in brass
Average UTS o f coating
Average elongation (%)
A t first cracks (kN mm -2)
A t failure o f test piece (kN mm -2)
A t first A t failure o f cracks test piece (kN mm -2) (kN rnm -2)
11.1 20.6 25.0 28.7
0.194 0.214 0,225 0,225
0,290 0,290 0,298 0.300
1.22 1.13 1.04 1.05
0.836 0.970 0.896 0.841
B
Z
A
D EXTENSION Fig. 7. L o a d - e x t e n s i o n curves for brass (
(---).
% ) a n d for an N i - C o - p l a t e d brass s p e c i m e n
noticeable elongation at an extension 0D. No test result was accepted unless the Ni-Co alloy and brass remained bonded t h r o u g h o u t the test. The UTS of the plated test piece was calculated using the equation UTS = P m a x / A o where Pmax is the m a x i m u m load and A 0 is the original cross-sectional area of the specimen. The true UTS was calculated by dividing the breaking load by the cross-sectional area at failure. The UTS of the deposit was calculated by dividing the load CA by the cross-sectional area of the coating. The UTS of the brass test piece was determined by dividing the load DA by the original cross-sectional area of the brass before it was coated. During the tests, the first cracks in the coating were recorded and elongation at t h a t stage was measured as reported. The results in Table 5 show t h a t at specimen failure the average UTSs of Ni-Co alloy electrodeposits, obtained using ultrasonic agitation, were slightly higher than those for the electrodeposits obtained by air agitation.
230 TABLE 6 Average tensile strength and ductility of test pieces plated with an N i - C o coating o b t a i n e d from a solution containing 110 g COSO4"7H20 1-1
Method of agitation
Average load at failure of plated test piece
Average UTS of plated test piece (kN m m -2)
(kN) Air Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 350 W
Average true UTS of plated test piece
Average elongation (%)
(kN m m -2)
6.97 7.14 7.19 7.24
0.311 0.328 0.330 0.334
0.458 0.468 0.473 0.485
21.2 24.2 28.2 32.0
The ductility of plated specimens was significantly higher when the Ni-Co coating was applied using ultrasonic agitation instead of air, as shown in Table 4. The tensile tests were repeated using samples plated in a solution containing 110 g COSO4-7H20 1-1 and the results are shown in Table 6. It is apparent that the tensile strength results are approximately the same as those for deposits obtained from the more dilute Ni-Co solution containing 10 g COSO4.7H20 1-1. However, the ductility results were lower than those from the dilute bath. The tensile strength results of air-agitated test pieces were marginally lower than those obtained by ultrasonic agitation but the ductilities of the test pieces were improved by applying ultrasonic agitation from an average of 21.2% using air agitation to an average of 32.06% by 350 W ultrasonic agitation. 3.2.2. N i - F e Triplicate results were obtained in all cases, as for Ni-Co deposits, and the results are shown in Tables 7 and 8. Erratic values were obtained for the UTS of N i - F e coatings obtained using ultrasonic agitation, as shown in Table 8. The average ductility value of the plated specimens was 22% when air agitation was used but increased to 34.8% when 350 W ultrasonic agitation was used, as shown in Table 7. TABLE 7 Average tensile strength and ductility o f test pieces plated with an N i - F e coating
Method o f agitation Air Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 350 W
Ave rage elongation
Average load at failure
Average UTS
Average true UTS
(kN)
(kN m m -2)
(kN m m -2)
(%)
6.38 6.83 6.80 7.15
0.284 0.305 0.303 0.319
0.456 0.466 0.470 0.475
22.0 28.5 31.1 34.8
231 TABLE 8 Average tensile s t r e n g t h and ductility o f p l a t e d N i - F e coatings d e t e r m i n e d in situ o n brass s u b s t r a t e Method o f agitation
First cracks Average load (kN)
Air 4.41 Ultrasonic 100 W 4.70 Ultrasonic 200 W 4.90 Ultrasonic 350 W 4.92
Average stress in b r a s s
Average UTS o f coating
Average A t first elongation cracks (%) (kN mm -2)
A t failure o f test piece (kN mm -2)
A t first cracks (kN mm -2)
A t failure o f test piece (kN mm -2)
1.6 5.7 6.2 9.9
0.25 0.27 0.28 0.29
1.37 1.44 1.23 1.45
1.00 1.05 0.70 0.95
0.139 0.148 0.168 0.159
4. Discussion
4.1. Efficiency of ultrasonic equipment After consultation with the manufacturers of the ultrasonic equipment it was estimated t h a t the efficiency of ultrasonic power in the plating tank during electrodeposition is reduced by a total of about 15% as a result of the damping effect of the water film (5%) and the plastic tank (10%). The values given in all tables and graphs relate to the nominal o u t p u t from one transducer. Two transducers were used but to obtain the actual power input the total nominal input should be reduced by approximately 15% as indicated above. However, the values quoted give a comparative measure of ultrasonic intensity. 4.2. Statistical analysis o f hardness results To obtain information on the accuracy of the hardness measurements an analysis of variance was performed on two separate sets of hardness data. In each case ten hardness measurements were made on each of ten hardness indentations, i.e. a total of 100 measurements were made. The detailed results have been reported by Mahmood [3]. They show that experimental errors in hardness testing can be ignored and t h a t averages taken on a particular sample give accurate results. The statistical analysis of hardness results indicates t h a t significant increases in hardness were achieved by the use of ultrasonic agitation instead of air agitation. Since average hardness measurements were shown to be very accurate any average hardness values differing by more than about ten can be said to come from different statistical populations. At an intensity of 350 W the increase in average hardness for deposits studied ranged between 65 and 97 HV. The increases in average hardness are shown in Table 9. 4.3. Effect o f ultrasonic agitation on hardness Various theories have been proposed to explain the p h e n o m e n o n of increased hardness due to the use of ultrasonic agitation instead of conven-
232 TABLE 9 Increase in average hardness using 350 W ultrasonic agitation instead of air agitation (pH 4; current density, 4 A dm-2; temperature, 55 °C for all solutions except 68 °C for Ni-Fe) Plating solution
Increase in hardness (HV)
Ni-Co Ni-Co Ni-Co Ni-Co Ni-Co Ni Co Ni-Fe
80 92 97 90 75 65 78 90
(10 g COSO4"7H20 1-1) (35 g COSO4"7H20 1-1) (60 g COSO4"7H20 l 1) (85 g COSO4"7H20 1-1) (110 g COSO4-7H20 1-1)
tional means of agitation. These theories are based on the production of small-grained deposits, work-hardening effects, an increase in dislocation density and internal stress. For alloy deposition the situation is further complicated by changes in the deposit composition due to the influence of ultrasonic agitation. 4.4. D e p e n d e n c e o f hardness o n grain size Kenahan et al. [4] have shown that ultrasonic agitation produced a
significant increase in hardness in both C u - Z n and Cu-Sn alloy deposits; the hardness of C u - Z n increased from 216 HV without ultrasonic agitation to 250 HV with ultrasonic agitation and the Cu-Sn alloy deposit increased from 247 to 315 HV with ultrasonic agitation. They related the increase in hardness to the fine grain size. Similarly Kochegin and Vyaseleva [5] reported that nickel deposits formed in an ultrasonic field were 60% harder than those produced using air agitation. Kochegin and Vyaseleva [5] showed that, with ultrasonic agitation below the cavitation threshold, the variable sound pressure of the ultrasonic field became the controlling factor in any property change. Under these conditions the cathode becomes passivated, and a smaller grain size results in the deposit, which gives an increase in hardness. When the ultrasonic agitation is accompanied by cavitation, the cathode is depassivated. In the present work cavitation was taking place. Walker and Benn [6] showed t h a t the microhardness of copper deposits obtained from an acid sulphate solution increased with increasing current density and decreased with increasing bath temperature. Both these factors affect the grain size, fine structures being obtained with a high current density and a low temperature. Ultrasonically agitated plating baths gave harder deposits with finer grain size than those from magnetically stirred baths. Macnaughton and Hothersall [7] suggested that the hardness was connected with grain size and, for a given grain size, differences in the hardness could be due to different packing within the crystal lattice.
233
In the present work it was revealed by transmission electron microscopy that the grain size of Ni-Co deposits was larger using ultrasonic agitation than with air [3]. Ultrasonic agitation did n o t appear to have much effect on the grain size of the bright Ni-Fe electrodeposits. Changes could hardly be detected even when a very high magnification of 500 000× was used. The very fine grain size of this alloy deposit was caused mainly by the organic addition agents in the solution. Kozan [8] reported that ultrasonic agitation promotes a generally larger grain structure throughout the thickness of the deposits obtained from the Watts nickel solution. The hardness varied from 245 HV using conventional agitation to 350 HV using ultrasonic agitation. These values are in fairly close agreement with results obtained in the present work for Watts nickel deposits. The values given in Table 2 are 247 HV for air agitation and 312 HV for ultrasonic agitation at 350 W. Kozan did not report the cobalt content of his deposits but in the present study the deposits contained some cobalt although extensive solution purification was carried out. However, both deposits had almost the same cobalt content, 5.88% for air and 5.7% for ultrasonic agitation. It is apparent that hardness is affected by factors other than cobalt content and fine grain size. Table 9 shows that the average hardness of cobalt deposits produced using ultrasonic agitation (350 W) is 78 HV greater than for deposits produced using air agitation. There was no effect on hardness due to reduced grain size because deposits obtained by air agitation were finer grained than those obtained by ultrasonic agitation [3]. In general, the change in grain size depends on the metal being deposited, the experimental conditions such as the current density and the frequency and intensity of the ultrasonic field. There is considerable support for the argument that ultrasonic agitation causes an increase in grain size since it reduces concentration polarization which encourages the formation of large-grained deposits.
4.5. Dependence of hardness on work-hardening effect Recently Walker and Walker [9, 10] related the hardness of electrodeposits to the cavitation phenomena, which can lead to surface work hardening by shock waves generated by the collapse of vapour bubbles which can cause severe erosion of the electrode surface. Walker [10] reported t h a t the hardness of annealed nickel increased from 110 to 225 HV after ultrasonic b o m b a r d m e n t for 1500 min and then softened to 190 HV when stored at room temperature. He explained that the decrease in hardness at room temperature was due to recovery of point defects in the work-hardened surface layer. The increase in hardness of the nickel electrodeposited using ultrasonic agitation is permanent, however, and no softening was observed during storage. This permanency is probably due to the fact that metal is constantly being plated onto the work-hardened cathode and the whole deposit is affected in this manner. In this form the point defects are not considered to be able to diffuse through the deposit to the surface and result in softening.
234 The hardnesses of Ni-Co deposits of varying composition plated using ultrasonic agitation during the present investigation were redetermined after 2 years' storage at room temperature. No softening during storage was detected which supports Walker's theory about the probable permanency of hardness due to work hardening.
4.6. Dependence o f hardness on dislocation density Hofer and Hintermann [11] found that three factors seemed to influence the hardness of copper deposits: the fineness of the grains, the dislocation density and the pinning of the dislocations by impurities. Deposits with the smallest grain size and the highest strain had the highest hardness value. Strengthening is brought about by obstructing the movement of dislocations; hence the obstacle causes a stress concentration factor. It is most probable that ultrasonic agitation causes an increase in dislocation density due to different growth mechanisms during deposition. 4.7. Dependence o f hardness on internal stress One cause of internal stress in electrodeposits is the incorporation of hydrogen produced if the cathode current efficiency for metal deposition is less than 100%. Szlaraska-Smalowska and Smalowski [ 12] have shown that the stress in copper deposits obtained from a pyrophosphate bath can be as high as 68 × 10 -3 kN mm -2. Walker and Clements [13] have reported that the stress in copper deposits, plated at various current densities, was reduced by as much as 50% when ultrasonic agitation was used. The results of cathode current efficiencies for Ni-Co and N i - F e obtained during the current research programme were generally very close to 100% (96%- 98%) using ultrasonic agitation at 350 W. As a result of the high efficiency only a small a m o u n t of atomic hydrogen was produced. In addition, the production of gaseous hydrogen from atomic hydrogen at the cathodic surface is assisted by ultrasonic agitation. This decreases the tendency for atomic hydrogen to be incorporated in the metal and reduces the internal stress of the deposit. Streaky diffraction patterns were obtained from cobalt deposits produced using air agitation but lower stress was indicated by diffraction patterns from deposits produced using ultrasonic agitation.
4.8. Significance o f factors affecting hardness On consideration of the results obtained for Ni-Co and N i - F e deposits, it seems that fine grain size, composition and internal stress do not make a major contribution to increased hardness when ultrasonic agitation is used. It is more likely that it is due to a combination of an increase in dislocation density and the work-hardening effect of cavitation. It is of interest to note that the harder Ni-Fe alloy exhibited a greater increase in hardness than Ni-Co. A similar large increase of 100% in hardness has been observed by Petrov [14] for chromium obtained from a chromate electrolyte.
235
4.9. Effect o f cobalt content on hardness o f N i - C o alloys Harder deposits are advantageous in many applications and there is potential merit in using cobalt as the hardening agent. Endicott and Knapp [15] examined the influence of operating variables on alloy deposits produced from conventional nickel sulphamate solutions containing cobalt, and Ericson [16] reported an increase in hardness due to cobalt addition to a concentrated nickel sulphamate solution. Belt et al. [17] recently reported the effects of cobalt additions on the composition, hardness and ductility of deposits from the concentrated sulphamate electrolyte. This study showed that sound Ni-Co alloys could be deposited with values of hardness up to 525 HV, but the deposits always contained at least some tensile stress. By an appropriate reduction in the current density and the cobalt concentration in the solution, it is possible to produce low stress Ni-Co alloy deposits with a hardness in excess of 400 HV. Belt et al. [18] reported that the relation between the cobalt concentration in the solution, the cobalt content of the deposit and the hardness is n o t direct and simple. Deposits of relatively high cobalt content may be softer than those containing a smaller proportion of cobalt and, in the presence of small amounts of cobalt in solution, the hardness and cobalt content vary substantially with change in the current density. Table 10 shows that when air agitation is used the hardness of deposits produced from sulphate-chloride solution increased as the concentration of COSO4.7H20 in the solution increased. By increasing the cobalt content in the alloy deposit from 23.3% to 81.8% the average hardness increased from 265.6 to 308 HV. This can be related to the hardening effect of the cobalt content in the deposit as reported b y Endicott and Knapp [15]. By applying ultrasonic agitation at 350 W to the Ni-Co solutions with different cobalt
T A B L E 10 V a r i a t i o n in c o b a l t c o n t e n t a n d h a r d n e s s using e i t h e r air o r u l t r a s o n i c a g i t a t i o n in N i - C o s o l u t i o n s c o n t a i n i n g d i f f e r e n t c o n c e n t r a t i o n s of COSO4"7H20 (pH 4; c u r r e n t d e n s i t y , 4 A d i n - 2 ; t e m p e r a t u r e , 55 °C)
Concentration of COS04" 7H20 (g 1-1)
Agitation
Co content in deposit
Hardness (HV)
(%) 10 35 60 85 110 10 35 60 85 110
Air Air Air Air Air Ultrasonic Ultrasonic Ultrasonic Ultrasonic Ultrasonic
350 350 350 350 350
W W W W W
23.3 51.0 66.4 71.7 81.8 18.4 42.0 58.4 63.8 68.3
265.6 280 289 300 308 345 372 386 390 383
236
contents using the same plating conditions the cobalt content decreased from 23.3% to 18.4% and the deposit was 80 HV harder using the solution with 10 g COSO4" 7H20 1-1. Consequently, ultrasonic agitation had a greater effect on hardening than on the cobalt content. The probable explanation for the change in composition of the Ni-Co deposits is the effects of ultrasonics on the relative rates o f transport of nickel and cobalt ions into the cathode film.
4.10. Effect of different types of agitation on Ni-Fe alloy deposition The different methods of agitation used to study their effects on composition and hardness were as follows: (i) air agitation; (ii) ultrasonic agitation using different powers (20, 40, 100, 200 and 350 W); (iii) both air and ultrasonic agitation together with different power intensities (40, 100, 200 and 350 W); (iv) no agitation. The results are shown in Table 11. The use of air and ultrasonic agitation together did n o t show significant differences from the use of ultrasonic agitation alone. When no agitation was used the iron content of deposits was lower than when any form of agitation was used but the hardness was similar to that achieved using air agitation. The average hardness of Ni-Fe deposits increased from 523.2 HV with air to 635 HV using ultrasonic and air agitation. The average iron c o n t e n t decreased from 24.78% with air to 22.8% using the dual type of agitation at 350 W. It is apparent that the cobalt content of the deposits is reduced more than the iron content by the application of ultrasonic agitation.
T A B L E 11 Variation in c o m p o s i t i o n and h a r d n e s s using d i f f e r e n t t y p e s o f agitation (pH 4 ; t e m p e r a ture, 65 °C; c u r r e n t d e n s i t y , 4 A d m -2 unless o t h e r w i s e s t a t e d )
Agitation
Average Fe content
Hardness (HV)
(%) Air Air 8 A dm -2 Ultrasonic 20 W Ultrasonic 40 W Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 3 50 W Air a n d ultrasonic 40 W Air a n d ultrasonic 100 W Air and ultrasonic 200 W Air and ultrasonic 350 W No agitation 4 A dm -2 No agitation 8 A dm -2
24.7 20.3 19.4 20.1 22.5 23.5 26.0 19.9 21.7 19.6 22.8 18.2 16.8
523.2 538.8 527.3 561.9 588.5 602.1 623.4 559.4 555.1 610.0 635.0 518.0 502.0
237
4.11. Tensile strength and ductility The results obtained in the present work showed clearly t h a t by using ultrasonic agitation the ductility improved significantly but the UTS changed only slightly. For engineering applications it would be advantageous to use ultrasonic agitation so that deposits of good ductility could be produced without any significant loss of strength. One of the main reasons for the increase in ductility in both alloys is likely to be a decrease in the internal stress. From the diffraction patterns it was apparent that the deposits produced using ultrasonic agitation were less stressed than those produced using air [ 3 ]. The UTS results obtained, especially for Ni-Co, are in reasonable agreement with those obtained by other workers. The ductilities measured at first cracks, after ultrasonic agitation had been used, gave much higher values than those obtained by previous workers although the exact methods of measurement were n o t given in most cases. Brook [19] has reported a tensile strength of 0.74 kN mm -2 for Watts nickel plated at 4.3 A dm -2. Bailey et al. [20] obtained a value of 0.41 kN mm -2 for the tensile strength of a similar deposit and a ductility of about 30%. Deposits from a conventional sulphamate solution had a similar strength but a ductility of only 18%. McFarlen [21] has reported the behaviour of Ni-Co alloy deposits obtained from sulphamate solution containing organic additives. These additives contained sulphur which causes embrittlement although a tensile strength as high as 1.96 kN mm -2 was quoted at a cobalt content of 35.5%. Safranek [22] has reported tensile strengths and ductilities for a variety of Ni-Co baths as shown in Table 12. Levy [23] has reported that N i - F e alloy deposits obtained from sulphate-chloride electrolytes containing 7.5 g of sodium-l,3,5naphthalene trisulphonate per litre as a stress reducer had tensile strengths in the range 1.37 - 1.76 kN mm -2 and ductilities of 2% - 3%. Some of the values obtained in the present work for both nickel alloys are summarized in Table 13.
TABLE 12 The effect of composition variation on tensile strength and ductility of deposits obtained from different Ni-Co solutions using conventional agitation
Co content
Tensile strength
Elongation
(%)
( k N m m -2)
(%)
1 20 35 40 50 80
1.41 1.40 1.03 - 1.30 1.37 - 1.51 1.76 - 1.88 0.55
4 3 -6 1 - 1.5 2 -4 -1
Plating bath
Sulphate-chloride Sulphamate-bromide Watts Sulphamate-bromide Sulphamate Sulphamate
238 T A B L E 13 Average tensile s t r e n g t h a n d d u c t i l i t y o f p l a t e d Ni Co a n d N i - F e coatings
Composition
Agitation
Average UTS (kN m m -2) o f coating A t first crack
A t failure o f test piece
Elongation (%) A t first crack
A t failure o f test piece
Ni Co (10 g COSO47 H 2 0 1-1)
Air Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 350 W
1.22 1.13 1.04 1.05
0.84 0.97 0.90 0.84
11.0 20.0 25.0 28.7
37.1 41.2 42.5 45.5
Ni-Fe
Air Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 3 5 0 W
1.37 1.44 1.23 1.45
1.00 1.05 0.70 0.95
1.6 5.7 6.2 9.9
22.0 28.5 31.1 34.7
5. Conclusions (1) All plating solutions were operated at their optimum conditions using either air or ultrasonic agitation (13 kHz). The hardnesses of Ni-Co, Ni-Fe, nickel and cobalt all increased with increasing ultrasonic agitation. (2) The cobalt content of the deposits decreased with increasing power of ultrasonic agitation at constant current density but the iron content of the N i - F e deposits did not change very much with increases in the ultrasonic power. (3) The strength of the nickel alloy coatings was not affected significantly by the application of ultrasonic agitation but the ductility was increased. (4) Ultrasonic agitation instead of air should be of use for engineering applications since it enables harder and more ductile nickel alloys to be produced without loss of tensile strength.
Acknowledgments The authors wish to thank Ultrasonics Ltd. of Shipley for providing the ultrasonic equipment employed. T. R. Mahmood would also like to thank the State Organization for Technical Industries in Baghdad (A1-Yarmouk State Establishment) for financial support provided for the duration of this research project.
References 1 C. T. Walker a n d R. Walker, Electrodeposition Surf. Treat., 1 ( 1 9 7 2 - 1 9 7 3 ) 457. 2 C. B. K e n a h a n a n d D. Schlain, Effects o f u l t r a s o n i c s o n brass plating, Rep. o f Investigations, J a n u a r y 1961 (U.S. B u r e a u o f Mines).
239 3 T. R. Mahmood, Ph.D. Thesis, University of Aston in Birmingham, Birmingham, 1983. 4 C. B. Kenahan, D. Schlain and E. Chin, Rep. o f Investigations 6938, April 1967 (U.S. Bureau of Mines). 5 S. M. Kochegin and G. Y. Vyaseleva, Electrodeposition o f Metals in Ultrasonic Fields, Consultants Bureau, New York, 1966. 6 R. Walker and R. C. Benn, Electrochim. Acta, 16 (1971) 1081. 7 D. J. Macnaughton and A. W. Hothersall, Trans. Faraday Soc., 24 (1928) 387; 31 (1935) 1168. 8 T. G. Kozan, Plating (East Orange, N J), 49 (1962) 495. 9 R. Walker and C. T. Walker, Ultrasonics, 13 (2) (1975) 79. 10 R. Walker, Trans. Inst. Met. Finish., 53 (1975) 40. 11 E. M. Hofer and H. E. Hintermann, J. Electrochem. Soc., 112 (1963) 167. 12 Z. Szlaraska-Smalowska and M. Smalowski, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 6 (1958) 427. 13 R. Walker and J. F. Clements, Met. Finish. J., 16 (1970) 100. 14 Yu. N. Petrov, Appl. Electr. Phenom. (U.S.S.R.), 4 (1966) 21. 15 D. W. Endicott and J. R. Knapp, Jr., Plating (East Orange, NJ), 53 (1966} 43. 16 H. Ericson, Galvanotechnik, 57 (1966) 85. 17 K. C. Belt, J. A. Crossley and R. J. Kendrick, Interfinish, Proc. 7th Int. Metal Finishing Conf., Hanover, May 1968, Deutsche Gesellschaft fiir Galvanotechnik, p. 222. 18 K. C. Belt, J. A. Crossley and S. A. Watson, Trans. Inst. Met. Finish., 48 (1970) 133. 19 P. A. Brook, Plating (East Orange, NJ), 47 (1960) 1269. 20 G. L. J. Bailey, S. A. Watson and L. Winkler, 1st Annu. Conf. o f the Australasian Institute o f Metal Finishing, October 1969, Australasian Institute of Metal Finishing, p. 21. 21 W. T. McFarlen, Plating (East Orange, NJ), 57 (1970) 46. 22 W. H. Safranek, The Properties o f Electrodeposited Metals and Alloys, Elsevier, Amsterdam, 1973, p. 63. 23 E. M. Levy, Plating (East Orange, NJ), 56 (1969) 903.
219
E F F E C T OF ULTRASONIC AGITATION ON N i - C o AND N i - F e DEPOSITION T. R. MAHMOOD
State Organization for Technical Industries, Baghdad (Iraq) J. K. DENNIS and P. L. B A R R E T T
Department of Metallurgy and Materials Engineering, University of Aston in Birmingham, Birmingham B4 7ET (Gt. Britain) (Received January 16, 1984)
Summary The effects of ultrasonic agitation on deposition from two iron group alloy plating solutions, Ni-Co and bright Ni-Fe, have been studied. Comparison has been made with deposits plated from the same solutions using controlled air agitation. The ultrasonic equipment employed had a fixed frequency of 13 kHz b u t the power o u t p u t from each transducer was variable up to a maximum of 350 W. The effects of air and ultrasonic agitation on hardness, ductility, tensile strength and composition have been studied. The results obtained show that by increasing the ultrasonic power the hardness of both alloys was increased significantly and their compositions changed. The cobalt content of Ni-Co was decreased and the iron content of N i - F e increased. The ductility of coatings was increased by use of ultrasonic agitation b u t the tensile strength did not change very much. Ultrasonic agitation is an expensive means of agitating plating solutions and is worthwhile only if significant improvements in properties can be achieved. The novel feature of simultaneous improvement in hardness and ductility could have useful engineering applications.
1. Introduction The application of ultrasonic agitation in electroplating has received attention in recent years because of its effects on grain size, hardness, internal stress, brightness, porosity and reduction in plating time [1]. Relevant literature in this field provides conflicting results, probably due to the methods b y which ultrasonics are applied and because different frequencies and intensities have been used which makes comparison of results difficult. Most attention has been paid to single-metal deposits and only limited information has been published on the behaviour of alloy plating solutions. 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
220 Some of the early claims for the effects of ultrasonic agitation were rather exaggerated but it has been demonstrated that some definite advantages may be gained by using high frequency sound during the electrodeposition of alloys. These include deposition at higher than normal current densities, resulting in decreased plating times, improvement in physical properties [2], increased brightness, higher adhesion to the substrate, finer grain size, reduced internal stress and lower porosity. It has been found that ultrasonic fields can increase the rate of metal ion transfer within the electrolyte solution and so modify the properties of the deposit. The intensity and distribution of the sound field within a plating cell play an important role in determining the effects observed. It is the aim of the present investigation to study the effects of ultrasonic agitation during electrodeposition of two nickel alloys and to determine whether worthwhile improvements can be achieved.
2. Experimental procedure
2.1. Equipment The plating cell consisted of a plastic tank of volume 10 1. This was a suitable size to enable it to fit into the ultrasonic tank unit. The cathode panel 10 cm × 5 cm was clamped midway between the two anodes placed at either end of the bath. A three-phase rectified d.c. supply was used. Two series of experiments were carried out, one using ultrasonic agitation and the other using air agitation. The ultrasonic equipment consisted of two main parts: (i) a frequency generator; (ii) a tank unit. The frequency generator gave a m a x i m u m o u t p u t of 350 W per transducer at a frequency of 13 kHz. The transducers could be tuned and the output power varied. The tank unit consisted of a free-standing container housing two magnetostrictive transducers fixed in the b o t t o m of a stainless steel tank. Ledges were provided inside the tank to enable the use of a sheet of stainless steel as a platform on which to stand the plating cell. It was assumed after discussions with the manufacturers that a 15% power loss occurred. The plating cell was immersed in water to approximately three-quarters of its height in the ultrasonic tank.
2.2. Electroplating procedure Flat mild steel panels 10 cm × 5 cm were plated in nickel, cobalt, NiCo and Ni-Fe solutions in the plastic cell using ultrasonic agitation. The airagitated nickel, cobalt and Ni-Co panels were also plated in the same cell but the air-agitated N i - F e samples were plated in a 55 1 tank as described below. Most panels were plated at 4 A dm -z for 40 min and the rest at 8 A dm -2 for 20 min. These panels were used for both hardness determination and analysis.
221 Flat brass strip-type Hounsfield test pieces were plated in both alloy plating solutions in the 10 1 tank. Both modes of agitation were used and all samples were plated for 40 min at 4 A dm -2.
2.3. Nickel plating solution The solution used for the electrodeposition of nickel was based on the Watts bath. The solution was purified before use by treating with 20 g of activated carbon per litre to remove organic impurities and by plating at low current density (0.5 - 1.0 A dm -2) for several hours to remove metallic contamination. All other baths were treated in a similar manner. 2.4. Cobalt plating solution The bath used for the electrodeposition of cobalt was based on the Watts nickel-type bath, i.e. the nickel c o n t e n t was replaced by cobalt salts. The solution composition was as follows: cobaltous sulphate (COSO4- 7HEO), 150 g 1-1; cobaltous chloride (COC12"6H20), 22.5 g l-i; boric acid (H3BO3) , 1 5 g l 1. 2.5. Ni-Co sulphate-chloride plating solution The mixed sulphate-chloride bath for the deposition of Ni-Co is well known, being based on the Watts nickel bath. The composition of the solution chosen for the initial work was as follows: nickel sulphate (NiSO4" 7H:O), 290 g 1 1; nickel chloride (NiCl:'6H20), 40 g l-l; COSO4"7H20, 10 g 1-1, H3BO3, 40 g 1-1. Rolled nickel anodes were used and the concentration of cobalt in the solution was maintained by addition of COSO4" 7H20. In order to vary the cobalt c o n t e n t of deposits, further plating solutions were prepared with 35, 60, 85 and 110 g COSO4" 7H20 1-1. 2.6. Ni-Fe plating solution A proprietary bright-levelling Ni-Fe solution was prepared as recommended by the manufacturers. The solution chosen for the work was an N i - F e alloy plating solution consisting of the following: NiSO4" 7H20, 130 g 1-1; NiC12.6H20 , 100 g 1-1; H3BO3, 40 g 1-1; FeSOa. 7H20 , 12.5 g 1-1; complexing agent, 20 g l-l; saccharin, 2 g 1-1; initial brightener (proprietary solution of wetting agent, saccharin leveller and brightener), 10 ml 1-1. In addition to the standard steel panels plated in the 10 1 cell, others were plated in a 55 1 tank of N i - F e solution using vigorous air agitation. The reason for plating in this tank rather than the 10 1 tank was to keep the solution in o p t i m u m working condition. Not only was it possible to keep the metal ion concentration constant but by regular plating it was also possible to prevent excessive formation of ferric iron. The pH value of this solution was maintained within the range 3.8 - 4.2; this range was dictated by the need to keep the trivalent iron at a low level, since a high rate of formation of iron(III) was favoured by a high pH. If the
222 pH fell below 3.8 levelling was reduced. Nickel carbonate was used to raise the pH to its r e c o m m e n d e d value. If the. pH rose above 4.2 there was a loss of brightness in the low current density areas and the ductility was reduced. The pH was lowered to t he r e c o m m e n d e d value when necessary using dilute 20 vol.% sulphuric acid. The solution t e m p e r a t u r e was maintained at 68 °C because at temperatures below 63 °C there was a reduction in brightness, levelling and cat hode current efficiency. Temperatures above 70 °C caused harmful degradation p r o d u c t s to form as a result of stabilizer breakdown.
2. 7. Maintenance o f the bath The iron was present in the bath as ferrous (iron(II)) and ferric (iron (III)) ions. It was i m p o r t a n t that the ferric iron c o n c e n t r a t i o n did not exceed 40% o f the total iron c o n c e n t r a t i o n in the bath, otherwise loss of ductility and cath o d e current efficiency resulted. If the bath was operated at pH values higher than r e c o m m e n d e d , the ferric iron c o n c e n t r a t i o n had a tend e n c y to rise. Under normal operating conditions, the ferric iron concentration did n o t exceed 20% of the total iron in solution because ferric iron was constantly being reduced to ferrous at t he cathode. The brightening and levelling properties of the solution were maintained by regular additions of a single maintenance brightener at the recomm e n d e d rate. 2.8. Anodes 7 5 w t . % N i - 2 5 w t . % F e alloy in granular form was contained in a titanium anode basket which was enclosed in a well-washed c o t t o n anode bag. 2.9. Hardness Microhardness tests were carried out on m o u n t e d cross sections of the deposits cut from the centre of the mild steel panels. Ten readings were taken from each sample and the average was calculated. 2.10. Tensile tests The widths and thicknesses of specimens were measured before and after plating. A nominal 5 cm gauge length was scribed on t h e m and the edges c h a m f e r e d to remove the thick coating f r o m these high current density regions so that p r e m a t u r e edge cracking did n o t occur. The exact distance between the scribed crosses was measured accurately by a Vernier travelling microscope. The specimens were fixed into the grips of a Hounsfield t e n s o m e t e r and th e strain was applied manually. The elongation and load were recorded. An initial m e a s u r e m e n t was taken shortly after the appearance o f small cracks or o t h e r tiny defects in the deposit. The test piece was unloaded and the p e r m a n e n t elongation of t he gauge length measured together with the applied load. The specimen was reloaded and e x t e n d e d until final failure occurred. The final elongation was calculated and the m a x i m u m load recorded. Triplicate results were obtained in all cases.
223
2.11. Analysis The compositions of the deposits produced by the two agitation methods were determined by electron probe microanalysis, atomic absorption spectrophotometry or wet chemical analysis. 3. Results
3. I. Composition and hardness 3.1.1. Ni-Co deposits The effects of ultrasonic agitation on the average hardness and the cobalt c o n t e n t are shown in Figs. 1 and 2. After every 3 h of plating the Ni-Co solution was analysed by atomic absorption spectrophotometry and then replenished by adding COSO4- 7H20 to bring it back to its original concentration. Detailed examination of the results [3] showed that the hardness and cobalt c o n t e n t of the air-agitated deposits were approximately the same at 4 and 8 A dm -2. The hardness at 4 A dm -2 varied between 254 and 280 HV, and at 8 A dm -2 between 258 and 282 HV. The cobalt c o n t e n t at 4 A dm -2 was between 19.6% and 26.5% while at 8 A dm -2 it was between 19.4% and 24.8%. The deposits obtained from the Ni-Co solution using ultrasonic agitation showed that the hardnesses and cobalt contents of the deposits depend on the degree of ultrasonic agitation power and current density. At 4 A dm 2 the cobalt content in the deposit varied from 22.6% to 18.4% whilst the hardness varied from 265 to 345 HV because different power intensities (from 20 to 350 W) were transmitted to the 10 1 electroplating bath, but at
cg I /
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'
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224 23-
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lio
g/l
Fig. 3. Relation between the COSO4"7H20 concentration in solution and the cobalt content in the deposit: - - o - - , air agitation;--$--, ultrasonic agitation (350 W).
8 A d m -2 the c o b a l t c o n t e n t in t h e d e p o s i t varied f r o m 22.00% to 18.00% w h i l e the hardness varied f r o m 2 6 9 t o 3 5 8 H V . F r o m these results it is e v i d e n t that ultrasonic agitation had a far greater e f f e c t o n hardness than o n c o b a l t c o n t e n t in t h e d e p o s i t , a l t h o u g h there w a s greater scatter in t h e hardness results using ultrasonic agitation. T h e average hardness for air-agitated s a m p l e s at 4 A d m 2 was 2 6 5 . 6 H V and for t h e highest u l t r a s o n i c p o w e r o f 3 5 0 W at 4 A d m -2 was 3 4 5 HV.
225
Since air agitation gave an average cobalt c o n t e n t in the deposit of 23.3% and an average hardness of 265.6 HV whilst ultrasonic agitation power of 350 W gave an average cobalt content of 18.4% and an average hardness of 345.3 HV, it seems reasonable to assume that the increase in hardness is due to the ultrasonic agitation effect.
3.1.2. Effect o f change in solution composition Further work was carried out using plating solutions with different cobalt concentrations. It was apparent t h a t the small percentages of cobalt in the bath resulted in high percentages of cobalt in the electrodeposit. The relation between the cobalt contents in the deposits and the concentration of COSO4- 7H20 in solution is shown in Fig. 3.
3.1.3. Ni-Co solution containing 110 g COS04" 7H20 1-1 The cobalt c o n t e n t of the deposit using air agitation was initially 81.8% and dropped to 68.3% at 350 W ultrasonic agitation. The hardnesses obtained using different ultrasonic agitation intensities ranged from 296 to 383 HV. Deposits with hardnesses of 308 HV and 312 HV were obtained by applying vigorous air agitation at 4 A dm -2 and 8 A dm -2 respectively. The more intense was the ultrasonic agitation the harder were the electrodeposits, as shown in Fig. 4, and the greater the intensity of the ultrasonic agitation the lower was the cobalt content, after an initial rise at 40 W, as shown in Table 1. A comparison between the hardness results obtained from this solution and those from that containing 10 g COSO4" 7H20 1-1 using ultrasonic agitation is also illustrated in Fig. 4. 380-
O~
360-
11~/
370-
J fjS
350~.~ 340-
320-
//
~ 310290-300"__e~e ~
270• 260250 0 20 4b
~/,~
--
140
ULTRASONIC
200
AGITATION
3(~0
POWER
3:$0
Watts
Fig. 4. V a r i a t i o n in h a r d n e s s o f nickel, c o b a l t a n d N i - C o alloys w i t h degree o f u l t r a s o n i c a g i t a t i o n : - - © --, N i - C o (110 g COSO4"7H20 l-S); - - ~3 - - N i - C o (10 g COSO4"7H20 1-1); ---o--, c o b a l t s o l u t i o n ; - - " - - , dull nickel s o l u t i o n .
226 TABLE 1 Hardness and composition of Ni-Co alloy deposits obtained using air or ultrasonic agitation (pH 4 ; temperature, 55 °C) Agitation
Current density (A dm 2)
Co content (%)
Hardness (HV)
Air Air Ultrasonic Ultrasonic Ultrasonic Ultrasonic Ultrasonic
4 8 4 4 4 4 4
81.8 81.2 64.8 72.4 71.5 69.5 68.3
308 312 297 296 308 328 383
20 W 40 W 100 W 200 W 350 W
The plating solution contained 110 g
C o S O 4 ° 7H20
1-1.
3.1.4. N i c k e l d e p o s i t s F r o m t h e r e s u l t s s h o w n in T a b l e 2 it is a p p a r e n t t h a t s o m e c o b a l t w a s p r e s e n t as i m p u r i t y . E l e c t r o d e p o s i t e d n i c k e l r a n g e d in h a r d n e s s f r o m 2 4 7 t o 253 HV when deposited using vigorous air agitation and current densities of 4 a n d 8 A d m -2. H a r d e r d e p o s i t s , a b o u t 2 5 9 - 3 1 2 H V , w e r e o b t a i n e d u s i n g u l t r a s o n i c a g i t a t i o n o f d i f f e r e n t p o w e r i n t e n s i t i e s . A v e r a g e h a r d n e s s versus u l t r a s o n i c a g i t a t i o n p o w e r is i l l u s t r a t e d in F i g . 4.
TABLE 2 Hardness and composition of nickel electrodeposits obtained using air or ultrasonic agitation (pH 4; temperature, 55 °C; current density, 4 A dm -2 unless otherwise stated) Agitation
Ni content (%)
Co content (%)
Hardness (HV)
Air Air 8 A dm -2 Ultrasonic 20 W Ultrasonic 40 W Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 350 W
94.47 92.16 94.61 94.02 94.09 94.19 94.31
5.88 8.32 5.62 6.10 6.30 5.86 5.70
247 253 258.6 264.8 265.4 275 312
3.1.5. Cobalt d e p o s i t s The hardnesses of the electrodeposits obtained by using air agitation w e r e 2 8 4 H V a n d 2 8 7 H V u s i n g c u r r e n t d e n s i t i e s o f 4 A d m - 2 a n d 8 A d m -2 r e s p e c t i v e l y . A t 4 A d m -2 t h e h a r d n e s s r a n g e d f r o m 2 9 2 . 6 t o 3 6 1 . 9 H V u s i n g u l t r a s o n i c a g i t a t i o n o f d i f f e r e n t p o w e r i n t e n s i t i e s , as s h o w n in T a b l e 3 a n d F i g . 4.
227 TABLE 3 Hardness of cobalt electrodeposits obtained using air or ultrasonic agitation (pH 4;temperature, 55 °C) A g i t a tio n
Air Air Ultrasonic Ultrasonic Ultrasonic Ultrasonic Ultrasonic
20 W 40 W 100 W 200 W 350 W
3.1.6. Ni-Fe
Cu rre n t density
Co content
Ni content
(A dm -2)
(%)
(%)
4 8 4 4 4 4 4
99.87 99.70 99.58 99.68 99.70 99.68 99.65
0.21 0.33 0.44 0.43 0.35 0.41 0.45
Hardness
(HV) 284 287 294.6 292.6 304 322 361.9
deposits
T h e h a r d n e s s e s o f t h e N i - F e alloy e l e c t r o d e p o s i t s r a n g e d f r o m 523 t o 5 3 8 . 8 H V using v i g o r o u s air a g i t a t i o n a n d c u r r e n t densities o f 4 a n d 8 A d m -2. H a r d e r d e p o s i t s w e r e o b t a i n e d ( 5 2 7 . 3 - 6 2 3 . 4 H V ) using u l t r a s o n i c a g i t a t i o n o f increasing p o w e r i n t e n s i t y , as s h o w n in Fig. 5. T h e iron c o n t e n t o f t h e d e p o s i t using air a g i t a t i o n was 24.8%. W h e n u l t r a s o n i c a g i t a t i o n was a p p l i e d t h e iron c o n t e n t varied b e t w e e n 1 9 . 4 2 % at 20 W a n d 26% a t 3 5 0 W. T h e s a m p l e s w e r e a n a l y s e d b y a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y a n d e l e c t r o n p r o b e m i c r o a n a l y s i s . T h e results illustrated in Fig. 6 s h o w g o o d a g r e e m e n t .
600
5 0 0 - 1
0 26 4b
.
.
~00
ULTRASONIC
.
.
.
2bo
AGITATION
.
.
~0
-
.
-3~,0
POWER , Watts
Fig. 5. Variation in hardness of Ni-Fe alloys with degree of ultrasonie agitation.
228
30-
25-
z
f
"
----------~
20-
15
Z 10,
5"
2b 4b
Ibo
ULTRASONIC
2~o AGITATION
~o
3~o
POWER,
Watts
Fig, 6. Variation in N i - F e alloy composition with degree of ultrasonic a g i t a t i o n : - ©--, results determined by electron probe microanalysis; ---$--, results determined by atomic absorption spectrophotometry.
3.2. Tensile strength and ductility 3.2.1. Ni-Co The average results are given in Tables 4 and 5. Figure 7 shows a typical load-extension curve for a brass specimen, 0AB, and for an electroplated brass specimen, 0C. During the testing of the Ni-Co electrodeposited specimens, fracture was always preceded by cracking of the deposit and fracture usually started from one of the edge defects. Thus it appears probable that fracture initiates in the Ni-Co electrodeposit and if this is completely ruptured before failure starts in the brass the load carried by the brass will be no longer equivalent to AD but to CD, which is greater than the ultimate tensile strength (UTS) of the brass. The brass will then fail without further TABLE 4 Average tensile strength and ductility of test pieces plated with an Ni-Co coating obtained from a solution containing 10 g COSO4"7H20 1-1 and uncoated brass
Method of agitation
Average load at failure of plated test piece
Average UTS of Average true UTS plated test of plated test piece piece
(kN)
(kN mm -2)
(kN mm -2)
0.320 0.328 0.328 0.330 0.317
0.464 0.474 0.483 0.490 0.463
Air 7.10 Ultrasonic 100 W 7.32 Ultrasonic 200 W 7.31 Ultrasonic 350 W 7.33 Unplated brass 6.80 UTS, ultimate tensile strength.
Average elongation (%) 37.1 41.2 42.5 45.5 69.9
229 TABLE 5 Average tensile s t r e n g t h a n d ductility o f p l a t e d N i - C o coatings o b t a i n e d f r o m a s o l u t i o n c o n t a i n i n g 10 g COSO4"7H20 1-1 d e t e r m i n e d in situ o n brass s u b s t r a t e s
Method o f agitation
First cracks Average load (kN)
Air 5.43 Ultrasonic 100 W 5.76 Ultrasonic 200 W 5.88 Ultrasonic 350 W 5.94
Average stress in brass
Average UTS o f coating
Average elongation (%)
A t first cracks (kN mm -2)
A t failure o f test piece (kN mm -2)
A t first A t failure o f cracks test piece (kN mm -2) (kN rnm -2)
11.1 20.6 25.0 28.7
0.194 0.214 0,225 0,225
0,290 0,290 0,298 0.300
1.22 1.13 1.04 1.05
0.836 0.970 0.896 0.841
B
Z
A
D EXTENSION Fig. 7. L o a d - e x t e n s i o n curves for brass (
(---).
% ) a n d for an N i - C o - p l a t e d brass s p e c i m e n
noticeable elongation at an extension 0D. No test result was accepted unless the Ni-Co alloy and brass remained bonded t h r o u g h o u t the test. The UTS of the plated test piece was calculated using the equation UTS = P m a x / A o where Pmax is the m a x i m u m load and A 0 is the original cross-sectional area of the specimen. The true UTS was calculated by dividing the breaking load by the cross-sectional area at failure. The UTS of the deposit was calculated by dividing the load CA by the cross-sectional area of the coating. The UTS of the brass test piece was determined by dividing the load DA by the original cross-sectional area of the brass before it was coated. During the tests, the first cracks in the coating were recorded and elongation at t h a t stage was measured as reported. The results in Table 5 show t h a t at specimen failure the average UTSs of Ni-Co alloy electrodeposits, obtained using ultrasonic agitation, were slightly higher than those for the electrodeposits obtained by air agitation.
230 TABLE 6 Average tensile strength and ductility of test pieces plated with an N i - C o coating o b t a i n e d from a solution containing 110 g COSO4"7H20 1-1
Method of agitation
Average load at failure of plated test piece
Average UTS of plated test piece (kN m m -2)
(kN) Air Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 350 W
Average true UTS of plated test piece
Average elongation (%)
(kN m m -2)
6.97 7.14 7.19 7.24
0.311 0.328 0.330 0.334
0.458 0.468 0.473 0.485
21.2 24.2 28.2 32.0
The ductility of plated specimens was significantly higher when the Ni-Co coating was applied using ultrasonic agitation instead of air, as shown in Table 4. The tensile tests were repeated using samples plated in a solution containing 110 g COSO4-7H20 1-1 and the results are shown in Table 6. It is apparent that the tensile strength results are approximately the same as those for deposits obtained from the more dilute Ni-Co solution containing 10 g COSO4.7H20 1-1. However, the ductility results were lower than those from the dilute bath. The tensile strength results of air-agitated test pieces were marginally lower than those obtained by ultrasonic agitation but the ductilities of the test pieces were improved by applying ultrasonic agitation from an average of 21.2% using air agitation to an average of 32.06% by 350 W ultrasonic agitation. 3.2.2. N i - F e Triplicate results were obtained in all cases, as for Ni-Co deposits, and the results are shown in Tables 7 and 8. Erratic values were obtained for the UTS of N i - F e coatings obtained using ultrasonic agitation, as shown in Table 8. The average ductility value of the plated specimens was 22% when air agitation was used but increased to 34.8% when 350 W ultrasonic agitation was used, as shown in Table 7. TABLE 7 Average tensile strength and ductility o f test pieces plated with an N i - F e coating
Method o f agitation Air Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 350 W
Ave rage elongation
Average load at failure
Average UTS
Average true UTS
(kN)
(kN m m -2)
(kN m m -2)
(%)
6.38 6.83 6.80 7.15
0.284 0.305 0.303 0.319
0.456 0.466 0.470 0.475
22.0 28.5 31.1 34.8
231 TABLE 8 Average tensile s t r e n g t h and ductility o f p l a t e d N i - F e coatings d e t e r m i n e d in situ o n brass s u b s t r a t e Method o f agitation
First cracks Average load (kN)
Air 4.41 Ultrasonic 100 W 4.70 Ultrasonic 200 W 4.90 Ultrasonic 350 W 4.92
Average stress in b r a s s
Average UTS o f coating
Average A t first elongation cracks (%) (kN mm -2)
A t failure o f test piece (kN mm -2)
A t first cracks (kN mm -2)
A t failure o f test piece (kN mm -2)
1.6 5.7 6.2 9.9
0.25 0.27 0.28 0.29
1.37 1.44 1.23 1.45
1.00 1.05 0.70 0.95
0.139 0.148 0.168 0.159
4. Discussion
4.1. Efficiency of ultrasonic equipment After consultation with the manufacturers of the ultrasonic equipment it was estimated t h a t the efficiency of ultrasonic power in the plating tank during electrodeposition is reduced by a total of about 15% as a result of the damping effect of the water film (5%) and the plastic tank (10%). The values given in all tables and graphs relate to the nominal o u t p u t from one transducer. Two transducers were used but to obtain the actual power input the total nominal input should be reduced by approximately 15% as indicated above. However, the values quoted give a comparative measure of ultrasonic intensity. 4.2. Statistical analysis o f hardness results To obtain information on the accuracy of the hardness measurements an analysis of variance was performed on two separate sets of hardness data. In each case ten hardness measurements were made on each of ten hardness indentations, i.e. a total of 100 measurements were made. The detailed results have been reported by Mahmood [3]. They show that experimental errors in hardness testing can be ignored and t h a t averages taken on a particular sample give accurate results. The statistical analysis of hardness results indicates t h a t significant increases in hardness were achieved by the use of ultrasonic agitation instead of air agitation. Since average hardness measurements were shown to be very accurate any average hardness values differing by more than about ten can be said to come from different statistical populations. At an intensity of 350 W the increase in average hardness for deposits studied ranged between 65 and 97 HV. The increases in average hardness are shown in Table 9. 4.3. Effect o f ultrasonic agitation on hardness Various theories have been proposed to explain the p h e n o m e n o n of increased hardness due to the use of ultrasonic agitation instead of conven-
232 TABLE 9 Increase in average hardness using 350 W ultrasonic agitation instead of air agitation (pH 4; current density, 4 A dm-2; temperature, 55 °C for all solutions except 68 °C for Ni-Fe) Plating solution
Increase in hardness (HV)
Ni-Co Ni-Co Ni-Co Ni-Co Ni-Co Ni Co Ni-Fe
80 92 97 90 75 65 78 90
(10 g COSO4"7H20 1-1) (35 g COSO4"7H20 1-1) (60 g COSO4"7H20 l 1) (85 g COSO4"7H20 1-1) (110 g COSO4-7H20 1-1)
tional means of agitation. These theories are based on the production of small-grained deposits, work-hardening effects, an increase in dislocation density and internal stress. For alloy deposition the situation is further complicated by changes in the deposit composition due to the influence of ultrasonic agitation. 4.4. D e p e n d e n c e o f hardness o n grain size Kenahan et al. [4] have shown that ultrasonic agitation produced a
significant increase in hardness in both C u - Z n and Cu-Sn alloy deposits; the hardness of C u - Z n increased from 216 HV without ultrasonic agitation to 250 HV with ultrasonic agitation and the Cu-Sn alloy deposit increased from 247 to 315 HV with ultrasonic agitation. They related the increase in hardness to the fine grain size. Similarly Kochegin and Vyaseleva [5] reported that nickel deposits formed in an ultrasonic field were 60% harder than those produced using air agitation. Kochegin and Vyaseleva [5] showed that, with ultrasonic agitation below the cavitation threshold, the variable sound pressure of the ultrasonic field became the controlling factor in any property change. Under these conditions the cathode becomes passivated, and a smaller grain size results in the deposit, which gives an increase in hardness. When the ultrasonic agitation is accompanied by cavitation, the cathode is depassivated. In the present work cavitation was taking place. Walker and Benn [6] showed t h a t the microhardness of copper deposits obtained from an acid sulphate solution increased with increasing current density and decreased with increasing bath temperature. Both these factors affect the grain size, fine structures being obtained with a high current density and a low temperature. Ultrasonically agitated plating baths gave harder deposits with finer grain size than those from magnetically stirred baths. Macnaughton and Hothersall [7] suggested that the hardness was connected with grain size and, for a given grain size, differences in the hardness could be due to different packing within the crystal lattice.
233
In the present work it was revealed by transmission electron microscopy that the grain size of Ni-Co deposits was larger using ultrasonic agitation than with air [3]. Ultrasonic agitation did n o t appear to have much effect on the grain size of the bright Ni-Fe electrodeposits. Changes could hardly be detected even when a very high magnification of 500 000× was used. The very fine grain size of this alloy deposit was caused mainly by the organic addition agents in the solution. Kozan [8] reported that ultrasonic agitation promotes a generally larger grain structure throughout the thickness of the deposits obtained from the Watts nickel solution. The hardness varied from 245 HV using conventional agitation to 350 HV using ultrasonic agitation. These values are in fairly close agreement with results obtained in the present work for Watts nickel deposits. The values given in Table 2 are 247 HV for air agitation and 312 HV for ultrasonic agitation at 350 W. Kozan did not report the cobalt content of his deposits but in the present study the deposits contained some cobalt although extensive solution purification was carried out. However, both deposits had almost the same cobalt content, 5.88% for air and 5.7% for ultrasonic agitation. It is apparent that hardness is affected by factors other than cobalt content and fine grain size. Table 9 shows that the average hardness of cobalt deposits produced using ultrasonic agitation (350 W) is 78 HV greater than for deposits produced using air agitation. There was no effect on hardness due to reduced grain size because deposits obtained by air agitation were finer grained than those obtained by ultrasonic agitation [3]. In general, the change in grain size depends on the metal being deposited, the experimental conditions such as the current density and the frequency and intensity of the ultrasonic field. There is considerable support for the argument that ultrasonic agitation causes an increase in grain size since it reduces concentration polarization which encourages the formation of large-grained deposits.
4.5. Dependence of hardness on work-hardening effect Recently Walker and Walker [9, 10] related the hardness of electrodeposits to the cavitation phenomena, which can lead to surface work hardening by shock waves generated by the collapse of vapour bubbles which can cause severe erosion of the electrode surface. Walker [10] reported t h a t the hardness of annealed nickel increased from 110 to 225 HV after ultrasonic b o m b a r d m e n t for 1500 min and then softened to 190 HV when stored at room temperature. He explained that the decrease in hardness at room temperature was due to recovery of point defects in the work-hardened surface layer. The increase in hardness of the nickel electrodeposited using ultrasonic agitation is permanent, however, and no softening was observed during storage. This permanency is probably due to the fact that metal is constantly being plated onto the work-hardened cathode and the whole deposit is affected in this manner. In this form the point defects are not considered to be able to diffuse through the deposit to the surface and result in softening.
234 The hardnesses of Ni-Co deposits of varying composition plated using ultrasonic agitation during the present investigation were redetermined after 2 years' storage at room temperature. No softening during storage was detected which supports Walker's theory about the probable permanency of hardness due to work hardening.
4.6. Dependence o f hardness on dislocation density Hofer and Hintermann [11] found that three factors seemed to influence the hardness of copper deposits: the fineness of the grains, the dislocation density and the pinning of the dislocations by impurities. Deposits with the smallest grain size and the highest strain had the highest hardness value. Strengthening is brought about by obstructing the movement of dislocations; hence the obstacle causes a stress concentration factor. It is most probable that ultrasonic agitation causes an increase in dislocation density due to different growth mechanisms during deposition. 4.7. Dependence o f hardness on internal stress One cause of internal stress in electrodeposits is the incorporation of hydrogen produced if the cathode current efficiency for metal deposition is less than 100%. Szlaraska-Smalowska and Smalowski [ 12] have shown that the stress in copper deposits obtained from a pyrophosphate bath can be as high as 68 × 10 -3 kN mm -2. Walker and Clements [13] have reported that the stress in copper deposits, plated at various current densities, was reduced by as much as 50% when ultrasonic agitation was used. The results of cathode current efficiencies for Ni-Co and N i - F e obtained during the current research programme were generally very close to 100% (96%- 98%) using ultrasonic agitation at 350 W. As a result of the high efficiency only a small a m o u n t of atomic hydrogen was produced. In addition, the production of gaseous hydrogen from atomic hydrogen at the cathodic surface is assisted by ultrasonic agitation. This decreases the tendency for atomic hydrogen to be incorporated in the metal and reduces the internal stress of the deposit. Streaky diffraction patterns were obtained from cobalt deposits produced using air agitation but lower stress was indicated by diffraction patterns from deposits produced using ultrasonic agitation.
4.8. Significance o f factors affecting hardness On consideration of the results obtained for Ni-Co and N i - F e deposits, it seems that fine grain size, composition and internal stress do not make a major contribution to increased hardness when ultrasonic agitation is used. It is more likely that it is due to a combination of an increase in dislocation density and the work-hardening effect of cavitation. It is of interest to note that the harder Ni-Fe alloy exhibited a greater increase in hardness than Ni-Co. A similar large increase of 100% in hardness has been observed by Petrov [14] for chromium obtained from a chromate electrolyte.
235
4.9. Effect o f cobalt content on hardness o f N i - C o alloys Harder deposits are advantageous in many applications and there is potential merit in using cobalt as the hardening agent. Endicott and Knapp [15] examined the influence of operating variables on alloy deposits produced from conventional nickel sulphamate solutions containing cobalt, and Ericson [16] reported an increase in hardness due to cobalt addition to a concentrated nickel sulphamate solution. Belt et al. [17] recently reported the effects of cobalt additions on the composition, hardness and ductility of deposits from the concentrated sulphamate electrolyte. This study showed that sound Ni-Co alloys could be deposited with values of hardness up to 525 HV, but the deposits always contained at least some tensile stress. By an appropriate reduction in the current density and the cobalt concentration in the solution, it is possible to produce low stress Ni-Co alloy deposits with a hardness in excess of 400 HV. Belt et al. [18] reported that the relation between the cobalt concentration in the solution, the cobalt content of the deposit and the hardness is n o t direct and simple. Deposits of relatively high cobalt content may be softer than those containing a smaller proportion of cobalt and, in the presence of small amounts of cobalt in solution, the hardness and cobalt content vary substantially with change in the current density. Table 10 shows that when air agitation is used the hardness of deposits produced from sulphate-chloride solution increased as the concentration of COSO4.7H20 in the solution increased. By increasing the cobalt content in the alloy deposit from 23.3% to 81.8% the average hardness increased from 265.6 to 308 HV. This can be related to the hardening effect of the cobalt content in the deposit as reported b y Endicott and Knapp [15]. By applying ultrasonic agitation at 350 W to the Ni-Co solutions with different cobalt
T A B L E 10 V a r i a t i o n in c o b a l t c o n t e n t a n d h a r d n e s s using e i t h e r air o r u l t r a s o n i c a g i t a t i o n in N i - C o s o l u t i o n s c o n t a i n i n g d i f f e r e n t c o n c e n t r a t i o n s of COSO4"7H20 (pH 4; c u r r e n t d e n s i t y , 4 A d i n - 2 ; t e m p e r a t u r e , 55 °C)
Concentration of COS04" 7H20 (g 1-1)
Agitation
Co content in deposit
Hardness (HV)
(%) 10 35 60 85 110 10 35 60 85 110
Air Air Air Air Air Ultrasonic Ultrasonic Ultrasonic Ultrasonic Ultrasonic
350 350 350 350 350
W W W W W
23.3 51.0 66.4 71.7 81.8 18.4 42.0 58.4 63.8 68.3
265.6 280 289 300 308 345 372 386 390 383
236
contents using the same plating conditions the cobalt content decreased from 23.3% to 18.4% and the deposit was 80 HV harder using the solution with 10 g COSO4" 7H20 1-1. Consequently, ultrasonic agitation had a greater effect on hardening than on the cobalt content. The probable explanation for the change in composition of the Ni-Co deposits is the effects of ultrasonics on the relative rates o f transport of nickel and cobalt ions into the cathode film.
4.10. Effect of different types of agitation on Ni-Fe alloy deposition The different methods of agitation used to study their effects on composition and hardness were as follows: (i) air agitation; (ii) ultrasonic agitation using different powers (20, 40, 100, 200 and 350 W); (iii) both air and ultrasonic agitation together with different power intensities (40, 100, 200 and 350 W); (iv) no agitation. The results are shown in Table 11. The use of air and ultrasonic agitation together did n o t show significant differences from the use of ultrasonic agitation alone. When no agitation was used the iron content of deposits was lower than when any form of agitation was used but the hardness was similar to that achieved using air agitation. The average hardness of Ni-Fe deposits increased from 523.2 HV with air to 635 HV using ultrasonic and air agitation. The average iron c o n t e n t decreased from 24.78% with air to 22.8% using the dual type of agitation at 350 W. It is apparent that the cobalt content of the deposits is reduced more than the iron content by the application of ultrasonic agitation.
T A B L E 11 Variation in c o m p o s i t i o n and h a r d n e s s using d i f f e r e n t t y p e s o f agitation (pH 4 ; t e m p e r a ture, 65 °C; c u r r e n t d e n s i t y , 4 A d m -2 unless o t h e r w i s e s t a t e d )
Agitation
Average Fe content
Hardness (HV)
(%) Air Air 8 A dm -2 Ultrasonic 20 W Ultrasonic 40 W Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 3 50 W Air a n d ultrasonic 40 W Air a n d ultrasonic 100 W Air and ultrasonic 200 W Air and ultrasonic 350 W No agitation 4 A dm -2 No agitation 8 A dm -2
24.7 20.3 19.4 20.1 22.5 23.5 26.0 19.9 21.7 19.6 22.8 18.2 16.8
523.2 538.8 527.3 561.9 588.5 602.1 623.4 559.4 555.1 610.0 635.0 518.0 502.0
237
4.11. Tensile strength and ductility The results obtained in the present work showed clearly t h a t by using ultrasonic agitation the ductility improved significantly but the UTS changed only slightly. For engineering applications it would be advantageous to use ultrasonic agitation so that deposits of good ductility could be produced without any significant loss of strength. One of the main reasons for the increase in ductility in both alloys is likely to be a decrease in the internal stress. From the diffraction patterns it was apparent that the deposits produced using ultrasonic agitation were less stressed than those produced using air [ 3 ]. The UTS results obtained, especially for Ni-Co, are in reasonable agreement with those obtained by other workers. The ductilities measured at first cracks, after ultrasonic agitation had been used, gave much higher values than those obtained by previous workers although the exact methods of measurement were n o t given in most cases. Brook [19] has reported a tensile strength of 0.74 kN mm -2 for Watts nickel plated at 4.3 A dm -2. Bailey et al. [20] obtained a value of 0.41 kN mm -2 for the tensile strength of a similar deposit and a ductility of about 30%. Deposits from a conventional sulphamate solution had a similar strength but a ductility of only 18%. McFarlen [21] has reported the behaviour of Ni-Co alloy deposits obtained from sulphamate solution containing organic additives. These additives contained sulphur which causes embrittlement although a tensile strength as high as 1.96 kN mm -2 was quoted at a cobalt content of 35.5%. Safranek [22] has reported tensile strengths and ductilities for a variety of Ni-Co baths as shown in Table 12. Levy [23] has reported that N i - F e alloy deposits obtained from sulphate-chloride electrolytes containing 7.5 g of sodium-l,3,5naphthalene trisulphonate per litre as a stress reducer had tensile strengths in the range 1.37 - 1.76 kN mm -2 and ductilities of 2% - 3%. Some of the values obtained in the present work for both nickel alloys are summarized in Table 13.
TABLE 12 The effect of composition variation on tensile strength and ductility of deposits obtained from different Ni-Co solutions using conventional agitation
Co content
Tensile strength
Elongation
(%)
( k N m m -2)
(%)
1 20 35 40 50 80
1.41 1.40 1.03 - 1.30 1.37 - 1.51 1.76 - 1.88 0.55
4 3 -6 1 - 1.5 2 -4 -1
Plating bath
Sulphate-chloride Sulphamate-bromide Watts Sulphamate-bromide Sulphamate Sulphamate
238 T A B L E 13 Average tensile s t r e n g t h a n d d u c t i l i t y o f p l a t e d Ni Co a n d N i - F e coatings
Composition
Agitation
Average UTS (kN m m -2) o f coating A t first crack
A t failure o f test piece
Elongation (%) A t first crack
A t failure o f test piece
Ni Co (10 g COSO47 H 2 0 1-1)
Air Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 350 W
1.22 1.13 1.04 1.05
0.84 0.97 0.90 0.84
11.0 20.0 25.0 28.7
37.1 41.2 42.5 45.5
Ni-Fe
Air Ultrasonic 100 W Ultrasonic 200 W Ultrasonic 3 5 0 W
1.37 1.44 1.23 1.45
1.00 1.05 0.70 0.95
1.6 5.7 6.2 9.9
22.0 28.5 31.1 34.7
5. Conclusions (1) All plating solutions were operated at their optimum conditions using either air or ultrasonic agitation (13 kHz). The hardnesses of Ni-Co, Ni-Fe, nickel and cobalt all increased with increasing ultrasonic agitation. (2) The cobalt content of the deposits decreased with increasing power of ultrasonic agitation at constant current density but the iron content of the N i - F e deposits did not change very much with increases in the ultrasonic power. (3) The strength of the nickel alloy coatings was not affected significantly by the application of ultrasonic agitation but the ductility was increased. (4) Ultrasonic agitation instead of air should be of use for engineering applications since it enables harder and more ductile nickel alloys to be produced without loss of tensile strength.
Acknowledgments The authors wish to thank Ultrasonics Ltd. of Shipley for providing the ultrasonic equipment employed. T. R. Mahmood would also like to thank the State Organization for Technical Industries in Baghdad (A1-Yarmouk State Establishment) for financial support provided for the duration of this research project.
References 1 C. T. Walker a n d R. Walker, Electrodeposition Surf. Treat., 1 ( 1 9 7 2 - 1 9 7 3 ) 457. 2 C. B. K e n a h a n a n d D. Schlain, Effects o f u l t r a s o n i c s o n brass plating, Rep. o f Investigations, J a n u a r y 1961 (U.S. B u r e a u o f Mines).
239 3 T. R. Mahmood, Ph.D. Thesis, University of Aston in Birmingham, Birmingham, 1983. 4 C. B. Kenahan, D. Schlain and E. Chin, Rep. o f Investigations 6938, April 1967 (U.S. Bureau of Mines). 5 S. M. Kochegin and G. Y. Vyaseleva, Electrodeposition o f Metals in Ultrasonic Fields, Consultants Bureau, New York, 1966. 6 R. Walker and R. C. Benn, Electrochim. Acta, 16 (1971) 1081. 7 D. J. Macnaughton and A. W. Hothersall, Trans. Faraday Soc., 24 (1928) 387; 31 (1935) 1168. 8 T. G. Kozan, Plating (East Orange, N J), 49 (1962) 495. 9 R. Walker and C. T. Walker, Ultrasonics, 13 (2) (1975) 79. 10 R. Walker, Trans. Inst. Met. Finish., 53 (1975) 40. 11 E. M. Hofer and H. E. Hintermann, J. Electrochem. Soc., 112 (1963) 167. 12 Z. Szlaraska-Smalowska and M. Smalowski, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 6 (1958) 427. 13 R. Walker and J. F. Clements, Met. Finish. J., 16 (1970) 100. 14 Yu. N. Petrov, Appl. Electr. Phenom. (U.S.S.R.), 4 (1966) 21. 15 D. W. Endicott and J. R. Knapp, Jr., Plating (East Orange, NJ), 53 (1966} 43. 16 H. Ericson, Galvanotechnik, 57 (1966) 85. 17 K. C. Belt, J. A. Crossley and R. J. Kendrick, Interfinish, Proc. 7th Int. Metal Finishing Conf., Hanover, May 1968, Deutsche Gesellschaft fiir Galvanotechnik, p. 222. 18 K. C. Belt, J. A. Crossley and S. A. Watson, Trans. Inst. Met. Finish., 48 (1970) 133. 19 P. A. Brook, Plating (East Orange, NJ), 47 (1960) 1269. 20 G. L. J. Bailey, S. A. Watson and L. Winkler, 1st Annu. Conf. o f the Australasian Institute o f Metal Finishing, October 1969, Australasian Institute of Metal Finishing, p. 21. 21 W. T. McFarlen, Plating (East Orange, NJ), 57 (1970) 46. 22 W. H. Safranek, The Properties o f Electrodeposited Metals and Alloys, Elsevier, Amsterdam, 1973, p. 63. 23 E. M. Levy, Plating (East Orange, NJ), 56 (1969) 903.